The field to which our invention relates is that known as electrical discharge machining, hereinafter referred to as "EDM" in which material is removed from an electrically conductive workpiece by the action of electrical gap discharges between an electrode and the workpiece. For the process to operate, it is necessary that the workpiece be electrically conductive. A servo feed system is used to provide relative movement between electrode and workpiece during machining thus to maintain a relatively constant and optimum gap spacing as material is progressively removed from the workpiece.
A dielectric fluid is circulated and recirculated generally under pressure through the gap throughout the machining operation. For most reliable results, a power supply of the independent pulse generator type is utilized to provide machining power pulses of precisely controllable frequency and duty factor. A multivibrator particularly suitable for use as the pulse generator in an EDM power supply is described and shown in Bell, U.S. Pat. No. 3,809,848, issued on May 7, 1974, for "Digitally Controlled Power Supply for Electrical Discharge Machining Apparatus". This patent is of common ownership with the present application.
Various over current protection systems have been disclosed in the prior art which generally sense the gap condition and in response to a condition approaching gap short circuit either the D.C. power source for the EDM circuit or, in some cases, the multivibrator producing the machining power pulses is cut off.
It is a considerable problem, particularly as at higher frequencies to determine whether a danger point is being approached and at that point interrupt the machining operation. Prior arrangements have been to limit the current to the gap by automatically adjusting it for each preselected frequency and current magnitude combination. One such system of this type is described in Bell, U.S. Pat. No. 3,737,615, issued on June 5, 1973, for "Current and Frequency Control System for Electrical Discharge Machining". Most prior art over current protection systems simply measure and respond to average gap voltage and responsive to voltage drop below a predetermined level, the EDM machining is interrupted. A system of this type is shown in Bell, U.S. Pat. No. 3,746,826, issued on July 17, 1973, for "Short Circuit Protection System for Electrical Discharge Machining".